The fact that it's bag seed has little to no bearing on growth rate. You could say seed from a seedback will have more reliable genetics and less hermies than bagseed, fine. You could say that seedbank seed will produce a more potent plant than bag seed, ok. You could even tell me seed from a bank will display characteristics in flowering that no ordinary bagseed would, I'd keep my mouth shut. To say that a plant from a seedbank seed will show more vigor than a plant from bagseed though is a preposterous claim. To speak a little of the uniformity of marijuana as a species in early life, most indoor growers won't even be able to tell the difference between a sativa and indica until they start flowering. What do you people think that seedbanks have some special patent on "super marijuana" and unless you spend at least $50 on ten seeds that you'll end up with weak, sickly plants that will topple over if they somehow miraculously produce more than a gram of bud? Get real, all I ever see myself growing is bag seed because for the $50 you spend on ten seeds I got about 50 seeds and an 1/2 oz of weed for the same price. If the seeds are no good, oh well, there's always the next bag.
It's most likely your light, what type is it and how far is it from the plants?